Using the convertProfit function (http://zorro-trader.com/manual/en/pip.htm) I get only one tenth of the normal value as PIPCost for the SPX500.
Comparing SPX500 to US30, while they have the same PIPCost (when convertProfit is not used) US30 has about 10 times the RollOver cost so the convertProfit-PIPCost seems to be the correct value.
However, the PIPCost FXCM lists on its website (http://docs.fxcorporate.com/user-guide/ug-cfd-product-guide-ltd-de.pdf) is the normal value, which is kind of strange.

You're right. But from what I see, the example in the manual is correct and the displayed pip cost by FXCM is wrong. Very strange.

The SPX500 pip size is 0.1. So if you buy 1 lot of SPX500 at $1600, and the price rises by 1 Pip, you end up at $1600.1 and have won 0.1 $, or 0.07 EUR. That's also the result of the example, and that's what you get when trading. But FXCM lists the SPX500 pip cost for a EUR account as 0.7, not 0.07.

Does anyone have an idea of how to explain that discrepancy? Otherwise I'll have to ask FXCM to explain it to me, exposing myself as a newbie once more.

Nope, I'm sorry. I already asked them about the dicrepancy between price, PIPCost and Margin and I was told that one PIP of the SPX500(0.1) is worth one Dollar.
As this did not make much sense to me I started this thread.

Anyways, the documantation (http://docs.fxcorporate.com/user-guide/ug-cfd-product-guide-ltd-de.pdf) also states that the RollOver is the same as for the underlying currency, which seems not to be the case in Zorro.

One pip of SPX500 is definitely not worth one Dollar. And the rollover is also not the same as for the underlying currency.

I've just checked the pip cost in the FXCM API and indeed, it's now 0.07, just as the value returned from convertProfit. But 2 months ago it was 0.7. That's the value in AssetFix.dta that was downloaded back then. Possibly FXCM made a mistake and silently fixed it meanwhile in the API, but not yet in TS and on their website.